In United Kingdom Patent No. 2,144,244 B to Cooper and Howlett, positive longitudinal pitch rate is used to indicate high rotor load factors, such as during high speed highly banked turns. The pitch rate signal is shaped, and when air speed exceeds a threshold level, the shaped signal is added to the normal engine reference speed signal, such as to command a higher engine speed. Said patent also suggests that a signal derived by shaping the output of a vertical body axis accelerometer could be used to enhance engine speed during heavy rotor load maneuvers, either alone or in conjunction with the shaped pitch rate signal. The enhanced engine speed increases the lift capacity and, consequently, the stall margin of the rotor during high rotor load maneuvers.
It was found that using a shaped pitch rate signal tends to abruptly reduce the engine speed near the end of a high rotor load maneuver, inducing yaw transients which are highly undesirable at precisely the point where stability is desired, thereby adding significantly to pilot workload. Further, using such a shaped signal tends to cause the engine to achieve speeds in excess of reference speed under heavy load, thus causing a tendency towards exceeding maximum permissible torque in the engine. Therefore, the pilot is required to constantly monitor engine torque, and provide either load-relaxing inputs to the blade pitch controls or lower engine speed using the pilot's engine speed beeper during the maneuver to prevent exceeding maximum permissible torque in the engine. As used herein, the terms "beep", "beep-up", and "beep-down" are intended to refer to pilot initiated variation in the engine reference speed from 100% rated speed using the aforementioned manually operated pilot's engine speed beeper.
In response to the aforementioned problems, U.S. Pat. No. 4,998,202 to Walsh et al. provides an improved fuel control which monitors vertical acceleration as determined from a vertical body axis accelerometer, and in response to a high load factor, i.e., a high vertical acceleration, in excess of a threshold level, a bias signal is provided so that helicopter engine reference speed is rapidly faded up to a maximum engine speed. Thereafter, the increased engine speed is maintained for a fixed period to ensure that maneuvers can be completed before the engine is allowed to droop. At the completion of the fixed period, load factor status is again checked for exceeding a lower acceleration threshold. If the load factor is below the lower threshold, the bias signal is progressively decreased to effect a gradual reduction in rotor speed from the enhanced rotor speed. Further, to prevent an engine over-torque condition, the bias signal is nulled if the engine torque exceeds a maximum threshold level. The increase in engine reference speed above 100% reference speed during heavy rotor loading is known as "load factor enhancement" or "engine speed enhancement", which terms are used interchangeably hereinafter.
The load factor enhancement described in the '202 patent to Walsh et al. dramatically improves yaw and aircraft altitude stability following high acceleration maneuvers. Accordingly, the targeting capability, i.e., the ability to lock onto a target, is greatly improved following such maneuvers. However, such load factor enhancement has been found to be inadequate for certain maneuvers. For example, prior to performing a "gun run", i.e. a generally nose down or high pitch angle descent lasting on the order of ten to fifteen seconds until pull-up at the completion of the run, the pilot will typically perform an observation type run or fly over which is followed by a roll-out maneuver producing sufficient vertical acceleration to initiate load factor enhancement. Thereafter, during the gun-run descent, the load factor enhancement may terminate prior to the end of the maneuver, and not be activated for the pull-up at the completion of the run.
Similarly, a bob-up maneuver involves a situation wherein the aircraft is initially hovering nap of the earth, i.e., below tree tops or behind mountains to remain covert, and then rapidly ascends above the tree top to perform observation. During the bob-up maneuver, sufficient vertical acceleration may or may not be produced to initiate load factor enhancement. At the completion of the observation, a bob-down maneuver is performed to return to the nap of the earth. Depending on the severity of the bob-down maneuver, i.e., the rate of descent, the rotor will tend to droop, and if the load factor enhancement has terminated prior to completion of the bob-down maneuver, or if load factor enhancement was never initiated during the bob-up maneuver, the pilot is left in the precarious position of performing the maneuver with less than maximum power.